The cellular response to DNA damage is an early anti-cancer barrier that maintains the genomic integrity by activating the tumor suppressor p53. Previous studies show that ATF3, an immediate-early gene which is rapidly induced as a part of cellular response to a wide-range of DNA damage, promotes p53 activation by binding to the tumor suppressor. Whereas ATF3 expression is down-regulated in various cancers, loss of ATF3 impairs the DNA damage response and results in tumorigenesis. The goal of this proposal is to elucidate the role of ATF3 in regulation of the DNA damage response and tumor suppression with a focus on understanding of the mechanisms by which ATF3 regulates p53 activity in face of oncogenic challenges. Preliminary studies show that ATF3 regulates not only the stability but also the trans-activation activity of p53, and also discover a link of ATF3 with other p53 network proteins including E3 ubiquitin ligase MDM2 and histone acetyltransferases p300, Tip60 and hMOF. We therefore hypothesize that these interactions could be behind the mechanisms by which ATF3 regulates p53 tumor suppressor activity in the DNA damage response. In Aim #1, a hypothesis that the interaction between ATF3 and MDM2 contributes to p53 stabilization by regulating the binding of MDM2 to p53 and/or its catalytic activity towards p53 ubiquitination will be tested. In addition, we will characterize the contribution of the ATF3-MDM2 feedback loop to p53 regulation using Mdm2-knockout mice. In Aim #2, we will test a hypothesis that ATF3 regulates p53 trans-activation activity by promoting the interactions of p53 with its transcriptional co-regulators. Towards this aim, effects of ATF3 on p53 acetylation catalyzed by p300, Tip60 and hMOF, and subsequent epigenetic alterations and recruitments of transcriptional regulators that are essential for activation of p53- target gene expression will be determined. In Aim #3, we will assess the contribution of ATF3 to tumor suppression using genetically-engineered animal models. ATF3-deficient mice will also be bred and tested for its susceptibility to lung carcinogenesis induced by smoke carcinogens. Together these studies will render evidence supporting an indispensable role of ATF3 in p53-mediated DNA damage response and tumor suppression. Given that inadequate DNA damage response is a driving force for cancer initiation and progression, the fundamental new knowledge obtained from the proposed studies will establish ATF3 as a promising target for preventive and/or therapeutic treatments of cancer.